The present invention relates to scrambling television signals transmitted over air or via a cable or satellite environment. The invention is most effective as an improvement of the subject matter of the provisional patent application, xe2x80x9cMETHOD AND APPARATUS FOR IMPROVED HORIZONTAL AND VERTICAL OVERLAY SIGNALS FOR HIGHER CONCEALMENT IN MODERN TV SETSxe2x80x9d, Ser. No. 60/069815 (""815) filed on Dec. 16, 1997. However, the present invention can be used alone to provide concealment in the form of prohibitive darkening and/or lightening of the signal.
In the previous concealment systems, the concealment is done by causing a television picture being viewed on an unauthorized television (TV) set to tear horizontally and/or jitter vertically. In general, these effects are effective in creating an unviewable picture unless the viewing is authorized and the signal is descrambled. In a population of modern television sets, however, effectiveness is not always at the highest levels when applying the scrambling signal of such prior art techniques.
An object of this invention is to improve existing concealment systems by using an erroneous clamp pulse. This erroneous clamp pulse is removed and thus ignored by a decoder of an authorized playback device, i.e. TV set, but will produce excessive darkening (or lightening) of a picture supplied by unauthorized television sets. Because today""s modern television sets now use some form of back porch clamping (for example, a clamp pulse to reference a black level starts 4.7 microseconds after the leading edge of sync), modification of the clamping signal during the horizontal blanking interval can cause a desired darkening scrambling effect.
In the preferred embodiments, this erroneous clamp pulse or signal is distinguished from the copy protection signal described in U.S. Pat. 4,631,603 (""603) and/or 5,130,810 (""810) to J. O. Ryan, both incorporated by reference, by the following distinguishing features:
1) A narrowed and/or position modulated horizontal sync pulse.
2) A modified color burst signal which may include a narrowed or widened color burst envelope.
3) The timing of the erroneous clamp pulse in the present invention is based on the TV set""s clamp pulse (not a video cassette recorder""s (VCR""s) automatic gain control (AGC) sample pulse as generally the case), and the TV set""s clamp pulse is generally timed from the leading edge of sync. In the Ryan references, the copy protection signal is based on the timing after the trailing edge of sync. That is, in copy protection signals for VCRs, an AGC pulse is timed from the trailing edge of sync or pseudo sync and the VCR""s AGC sample pulse likewise is timed from the trailing edge of sync.
Another object of this invention is to improve on the effect of a vertical overlay signal by taking up fewer television lines in the vertical blanking interval (VBI) for added xe2x80x9cfakexe2x80x9d vertical sync pulses. By taking up fewer lines in the vertical blanking interval, more data lines in the VBI are available for other purposes.
A further object of the invention is to disclose a simpler way of inducing more vertical jitter and or rolling effects utilizing the xe2x80x9cfakexe2x80x9d vertical sync pulses.
To help clarify the invention over the above mentioned references, it is worthwhile to differentiate video xe2x80x9ccopy protectionxe2x80x9d techniques, such as disclosed in the ""603 and ""810 patents, from video xe2x80x9cscramblingxe2x80x9d techniques such as that of the present invention. To one skilled in the art of video engineering, video copy protection is defined as a system wherein a copy protected video signal is viewable with a minimum of or no visible artifacts, but where the playback of a recording of such a signal is not possible or produces a signal that has significantly degraded entertainment value. On the other hand, video scrambling is defined as a system wherein a video signal is made unviewable. A scrambled signal may be recordable, but unless it has been descrambled the playback of such a recording is still unviewable.
A primary feature of the present invention includes the technique of adding a pulse or signal, having a level around peak white, during or after the color burst signal, or a portion of the color burst signal. In this example, the color burst signal preferably has a narrower envelope than normal. In an embodiment in which the input video signal is previously scrambled as, for example, by horizontal sync modulation, the position modulated horizontal sync pulse preceding the narrower color burst is also narrowed. These techniques of the invention are predicated on the fact that most modern television sets use back porch clamping and use a horizontal oscillator and/or high voltage flyback pulse to form the clamp pulse. The clamp pulse is triggered by the leading edge of horizontal sync pulse and generally starts around 4.7 microseconds later. However, in a standard video signal, the back porch blanking level is approximately 4.7 microseconds after the leading edge of a horizontal sync pulse. Thus, if a pulse other than blanking level is placed around 4.7 microseconds after a leading edge of sync, then the television set will trigger (or clamp) on, and thus will display, an erroneous black level. This erroneous black level can cause the picture to darken abnormally when the pulse is higher than the normal blanking level, or brighten abnormally when the pulse is lower than the normal blanking level. Since extra concealment in a scrambled TV signal is the goal of the present invention, the abnormal or excessive darkening or lightening of the displayed scrambled picture caused by the present invention provides a highly effective scrambling technique wherein a displayed TV picture not only is torn horizontally and jittered vertically, but also is excessively darkened or lightened.
Therefore, a preferred embodiment of the invention includes the narrowing of both the horizontal sync and color burst envelope and then the adding of an erroneous clamp pulse (ECP) around peak white level after the narrowed color burst. Note that in one embodiment, the narrowed sync pulse and color burst, along with the added erroneous clamp pulse (ECP), are all being position shifted or modulated. In an alternative embodiment, the color burst envelope does not need to be shortened, but rather can be of normal or longer length. This is done by including at least a part of the color burst superimposed on the erroneous clamp pulse. In this instance, a decoder of the TV set can be made to have a shortened back porch clamp pulse not coincident with the erroneous pulse. To recover color burst, the decoder""s color burst gate pulse can occur after the trailing edge of sync because the associated chroma band pass filter will reject the erroneous pulse portion of the signal and will only pass through the color subcarrier frequency.
The present invention can be implemented alone as an effective scrambling system, or preferably can be combined with the techniques as described for example, in the ""815 provisional application of previous mention, or in U.S. Pat. Nos. 5,504,815, 5,438,620 or 5,058,157 (all incorporated by reference) for an even more enhanced scrambling signal. In the latter combinations, the invention causes darkening (or lightening) that is combined with horizontal tearing and vertical instability along with horizontal picture displacement and inverted video as described in the above patents. That is, the resulting scrambled signal will have horizontal tearing due to position (or shift) modulated horizontal sync edge or edges, vertical jittering due to deleting standard vertical sync pulses and replacing them with modulating vertical sync pulses, inverted video, and line position modulating on the video, as well as the darkening (or lightening) of the scrambled signal via the erroneous clamp pulse signals in the horizontal blanking interval, in accordance with the present invention.
Another alternative embodiment of this invention includes modifying a last portion of the color burst signal so as to cause the color subcarrier system in an unauthorized TV set to unlock and cause an erroneous color display.
Yet another alternative feature of this invention includes the generation of a known location for a decoder""s clamping system for example, a dedicated black level interval or a sync tip interval, and to incorporate sync/AGC pulse pair signals as disclosed in the Ryan ""603 and/or ""810 patents of previous mention, for even more darkening when combined with a scrambled signal. The use of position modulated (or shifted) sync/AGC pulse pair signals is motivated by the fact that certain TV sets respond to this type of signal by darkening completely (not to mention the horizontal tearing that also is present).
Another modification of the ""815 provisional application of previous mention, includes a unique method of utilizing fewer lines in the vertical blanking interval for the vertical overlay signal while providing more concealment by increased vertical instability.